Digital Applications

Transistors and Transistor Circuits

We're in Section 10-4 and this will be split into sections A and B. A will look at digital applications and B will look at analog applications. We're going to limit ourselves to just looking at the applications within this particular chapter. Entire books have been written on transistor applications because there are just so many of them. We're just going to be looking at just a small sampling of things that can be done with transistors.

First of all, digital applications. When a transistor is used as a switch it is either fully on or fully off. Transistor switches are common in digital circuits, industrial control circuits, burglar alarms, computer circuits and much more. A transistor switch used as a lamp driver is illustrated at the right. Here we have a computer and it's connected to a transistor and to a light. Recall that the transistor as a switch is either fully on or fully off. This would correspond to saturation and cutoff. Those are the terms that are used to describe that. In cutoff, there's no current. Saturation is maximum.

The industrial computer, the outputs would be--with a computer you're going to have two levels of outputs and typically the levels would be zero and five. Zero for a low and 5 volts for a high. If there are zero volts going to this configuration, then the biasing from the emitter to base junction is going to be zero and zero. If there are zero bolts across this diode, we don't have any current flows. If there's on current in the emitter, there's no current in the collector. This looks like an open switch. If we were to measure the voltage here, you would see 24 volts. The fuse that is negative. The transistor in this case essentially looks like an open switch because there is no voltage drop. There's a voltage drop across it but it's just the supply voltage here. This is not conducting and there is no voltage across the light, so it is off.

If we have a 5 volt input coming out of our computer, it will first see this base resistor here and it will see the resistance of the diode. The diode will drop 0.7 and this would drop 4.3 of that 5 volts if it's a 5-volt input. What would happen in this case is that now we have the proper biasing. The diode is going to turn on and this diode is going to conduct what we call saturation. It's going to conduct as hard as it possibly can. The biasing on the base-collector, it will also go into saturation. Usually, what you'll see here, remember, we've got zero here. We see 0.7 here.

Recall that this is just another diode here. This is just another diode and it's going to saturate, it's going to conduct as hard as it can. What you'll end up is 0.7 here then this is going to reach towards zero. What you'll usually see is maybe 0.1 to maybe 0.2 volts here. Now we have a situation where if we measure the voltage from here to here, we would be seeing maybe 0.2 volts. This would look like almost like a dead short. The result of that, if there are only 0.2 volts here, that means that will be 23.8 volts across this light. The light would illuminate.

One of the advantages of using a transistor in this configuration is that we have this industrial computer. The computer is not going to put out much current. Remember that the current that flows through the transistor is a function of what we call beta, well actually the relationship of the current between the collector and the base. Remember that beta varies greatly and beta could be, I'll just throw a number here, it could be 300. That would mean that the collector current would be 300 times greater than the base current. Remember that we said that when current flows through the emitter, for practical purposes we say that IE equals IC because such a tiny amount of that actually goes through the base.

Say we had 1 amp of current flowing from the emitter to base, that if the beta was 300 then the current that would load this computer would be 1 amp divided by 300 which would be very small. That is another reason why a transistor is used as an interface because it simply will not load down the source. It's telling it to turn on.

Integrated Logic Circuits

All modern computers are built around integrated circuits i.e. ICs. ICs are made from tens, thousands and even millions of integrated transistors. The pins on an IC connect to some of the interface transistors.

Bipolar. Modern ICS do not use the bipolar transistor as often as in the past, but their functions are still applicable. In fact, very frequently they're using the MOS technologies.

Input Circuit

Here we have input circuit. This circuit uses a PNP common collector transistor and Schottky diodes. This could be an input into a digital circuit. We are using the Schottky diodes here. What this is going to do is it's going to protect the input over here from high voltage. If the voltage is too high it will short the grounds through the Schottky diode.

D2 is a collector clamping diode. It keeps Q1 from going too deep into saturation. This allows the circuit to switch from saturation to cutoff more rapidly. What they found in transistors is they go into saturation too deeply it takes them a while to get out of saturation and into cutoff. This diode will keep that transistor from going too deep into saturation. The output will come out here and the output will be either a high or a low based on cutoff or saturation. Again, in this particular situation, if the transistor is cutoff then the full 5 volts would be felt right here. If the transistor is saturated, the voltage from here to here will probably about 0.2 volts, so the output would be about 0.2 right here because it is 0.2 here to here then you would have your output right there. The outputs in this would be probably about 0.2 and 5 volts. This would be the low. This would be the high.

Totem-Pole Output Circuit

Then we have a totem-pole output circuit. This tote-pole logic provides either a high or a low output. If T3 is on T4 is off and vice versa. Here we have this unique configuration and the idea here is that one transistor will be on the other is off. When that occurs the transistor that is on, remember we looked at saturation. If this one is on, we have only about 0.2 volts across it. Then if this a 5-volt logic, then the remainder of the voltage would be down here. If this is going to ground right here then that would give us our high.

The reverse situation, if this were conducting, we have about 0.2 volts here. This would be open. Most of our voltage would be dropped here. There would be about 0.2 volts here. That would represent our low. This is a totem-pole output circuit frequently used in the output stage digital circuits.

MOSFET: CMOS Inverter

Then we have the MOSFET and this is a CMOS Inverter. MOSFETs are used in the majority of ICs produced today for digital products. Both p- and n-channel devices are commonly used. If you look at this, here we have an input, here we have an output. This is an n-type CMOS and this is a p-type. What we'll see is zero volts in will cause Q2 to be cut off and Q1 to saturate. We have a zero bolt. If you have zero bolts coming in, Q2 will be cut off and this one will saturate.

With these e-mode MOSFETs, it will have the same phenomenon. When this one is cut off, it's going to represent an absolutely giant resistance. When this one is cut off, it's going to look like a short. With CMOS the resistance is so large here and the short is such a small drop here that to a fairly close approximation, the outputs are going to be either zero or five volts. The other, 5 volts in will cause Q2 to saturate, this would be shorted and Q1 to be cut off. This is the same idea as the totem-pole logic. One is on and the other is off.
The transfer curve in your text shows the crossover points are about 2.5 volts. You have your input coming in probably zero and 5 volts or zero and 3.5 or depending on the digital logic level. The crossover points are about 2.5 volts. Your text doesn't mention this but CMOS technology makes very low current devices possible. Things like digital watches and satellites use CMOS technology because they need to operate on extremely low currents. You may have observed with a digital watch that the battery seems to last forever and that's because it is built with CMOS technology that operates on extremely low current.

Hybrid

Then there are hybrids. Bipolar technologies are generally faster than MOSFET technology but consumes substantially more power. The bipolar are faster but they use a lot of power. The MOSFETs are very low current. Bipolar devices also drive greater loads at higher speeds than MOSFETs. Bipolar is more suitable for higher loads. MOSFET devices can be built with higher densities. Manufacturers often combine the two technologies within an IC. Within a given IC the MOSFET, you're able to put a lot of them on a chip. You can pack them at higher densities than you can with bipolar. Manufacturers like Intel will often have what they call a hybrid which will be a combination of MOSFET technology and bipolar in their digital technologies.

Memory Devices

All computers make extensive use of semiconductor memories. Transistors (bipolar and MOSFET) are used as memory elements. MOSFET memory is the more common. Each bit of binary data is represented by a cut off or saturated MOSFET. Notice that every bit of memory data is represented by either a cutoff or a saturated MOSFET. The MOSFET you'd have eight tiny MOSFETs to represent one bite of data.

Also, memory devices using bipolar and MOSFET, when you look on a computer and you see the cache, the L1 and L2 are built out of this type of device. This part is usually built right into the microprocessor. You'll see this in memory in your PC.

This has just been a really brief look at some of the applications for transistors. We looked at memory. We looked at hybrid, MOSFETs, CMOS, totem-pole output. We've looked at integrated logic circuits. That concludes our look at digital applications. In the next section, we're going to be looking at linear applications for transistors.